In recent years, portable equipment has become smaller in size and lighter in weight, and accordingly there has been increasing demand for lithium-ion secondary batteries having light weight and high energy density. While such batteries have the above-mentioned advantages, they tend to deteriorate when they are left at high temperatures because they include electrolyte containing an organic solvent. In particular, when the battery is left at high temperatures in a state in which it is charged to 100%, the electrolyte is decomposed on the surface of the positive electrode to generate gases, resulting in increasing the inner pressure. As a result, the battery may be expanded and deformed.
In order to solve this problem, for example, Japanese Patent Unexamined Publication No. 2002-56900 discloses the following configuration. That is to say, a temperature sensor is fixed on the outer surface of a battery, and when a temperature of the battery is not lower than a predetermined temperature and a voltage is not lower than a predetermined voltage, the battery is discharged through a discharge resistor inside a battery pack. Thereby, the voltage of the battery is reduced. However, this configuration does not have a function of displaying an error message. Therefore, a user of portable equipment does not notice that the remaining capacity of the battery is reduced because the battery is discharged in order to avoid an abnormality. When the portable equipment is a portable telephone, the user may simply not recognize that the deterioration of the battery shortens the standby time and therefore the battery needs to be replaced by a new one. Furthermore, the user may repeat the misrecognition after the battery is replaced with a new one because there is no opportunity to know what the abnormal state of a battery is like.
Furthermore, for example, Japanese Patent Unexamined Publication No. 2000-78250 discloses a portable telephone in which when an abnormality associated with a battery occurs during the standby time, the occurrence of the abnormality is displayed on a display portion. This portable telephone determines a state of the battery and displays the state during the standby time, that is, when the power-supply is on. Therefore, for example, when the portable telephone is left in an abnormal state with power supply turned off, a state of the battery cannot be diagnosed and an error message cannot be displayed. Furthermore, since only a function of informing a user that a battery is abnormal is described, a user cannot know whether or not this abnormality can be avoided.